In computer peripheral systems, as well as other applications, data is stored on a magnetizable medium such as a coated disc or tape by the proper energization of a magnetic head. The magnetic head includes a coil of wire surrounding a magnetic core which is disposed close to the medium so as to impress a portion of the magnetic flux generated (as a result of an electric current flowing through the windings) on the area of the magnetic medium immediately adjacent an air gap across which the flux can be varied. For recording, the direction of the flux is reversed to indicate a selected binary code.
As disclosed in the U.S. Pat. No. 2,927,304 issued on Mar. 1, 1960, in the name of James H. Paquin, the coil can be center-tapped and by switching between the ends of the coil halves and the center tap, the direction of flux generated can be reversed while maintaining the overall current level substantially constant. For reading back the information, the magnetic head is moved along the medium such that the coil is passed through the flux field caused by the medium for effecting an electric potential across the coil. This voltage is detected and interpreted to read the recorded information.
Naturally the magnitude of the voltage resulting in reading back the information is much less than that resulting from recording information. In the readback process a differential voltage in the coil is detected which results from the direction of magnetization of the medium over which the head is passed. Such signals are routinely quite small, i.e. on an order of magnitude of only one ten-thousandth that of the write signal. In the usual case, the readback signal is on the order of 1-10 millivolts thereby requiring substantial amplification prior to use and detection. It is therefore imperative that the write current circuit be as impedance and distortion-free as possible for proper detection of the signal.
However since the same coils are used for both reading and writing the information and since switching means must be provided for both selecting the direction of energization of each individual coil and for selecting the individual coils from a number of magnetic heads, there exists the possibility of considerable noise generation in the circuit and dissipation of the read signal. For instance in the switching of the current from one coil half to another for the writing process, there are generated voltage spikes in the write circuit which can pass into the read circuit. Such pulses of energy can serve to permanently damage any transistor amplifiers in the read circuit. Routinely a transistor can only withstand approximately 5 volts of reverse base-to-emitter voltage without breaking down and possibly causing permanent damage to the device. Frequently the voltage spikes resulting from the switching within the write circuit can reach 7 to 10 volts.
In addition by subjecting the read circuit to the back voltage of the write circuit, the speed of operation of the overall circuit is slowed. After a writing operation, the read amplifier and circuit in past devices must be afforded time to recover from being back biased before the reading sequence can be initiated. Thus the reaction time for the circuit is lengthened.
Thus in the past there has been utilized a diode limiter circuit which limits the signal swing the read circuit receives from the write circuit during the time the overall circuit is in the write mode for the protection of the amplifier. However, such diodes interject both noise and cause losses in the circuit so as to diminish the read voltage and inhibit the ability of the system to detect the data. It is the primary purpose of this invention to provide a circuit which diminishes the effect of the limiter circuit on the read voltage.